This invention relates to the valve art, and, more particularly, to ball valves.
The invention is particularly applicable to a new and improved ball valve and seat assembly for a valve of the type having a so-called "floating ball" and will be described with particular reference thereto. However, it will become readily apparent to those skilled in the art that the invention is capable of broader applications and could be adapted for use in other types and styles of valves.
Ball valve constructions in commercial use typically employ annular seats or seat rings formed of a resilient and deformable plastic for sealing engagement with the ball. A pair of such seat rings are positioned adjacent inlet and outlet openings. The ball itself is mounted for a slight amount of free movement or shifting axially of the seat when the ball is in a valve closed position under fluid pressure conditions. Such shifting causes the ball to act against and flex the downstream seat ring to enhance its sealing engagement with the ball. The amount of such flexing varies in accordance with the fluid pressure involved.
The floating ball concept has been successfully used in many arrangements and designs for small ball valves, and for lower pressure ball valves. As fluid pressure increases and causes downstream shifting of the ball, the ball is moved away from the upstream seat and all the force is applied to the downstream seat. Since the seat comprises an annulus of a smaller area, seat stress is always greater than the fluid pressure. In addition, as the valve size itself increases, the hall force increases as a function of the square of the seal diameter. However, typically it is impractical to increase the annular seat area at the same rate. Therefore, in a large valve or at high pressure, the seat stress can reach levels that may crush ordinary plastic type seats.
An overall objective of ball valve seat designs is to obtain a valve which will seal at both low pressure and high pressure and that will not require an unreasonable amount of operating torque for rotating the ball member. The floating ball type of concept is particularly advantageous in that it facilitates elimination of the more expensive trunnion mounted type of ball member as well as more complex pressure actuated valve seats.
Various forms and types of ball valve seat designs have heretofore been suggested and employed in the industry, all with varying degrees of success. It has been found that the defects present in most prior ball valve seat designs are such that the devices themselves are of limited economic and practical value.
Prior ball valves employing an elementary seat have included a seat design typically comprised of annular resilient plastic seats merely compacted between the ball member and opposed end fittings. The ball engaging surface of each seat includes a contour which matably engages the ball member periphery upon compacting assembly.
Elementary seat designs suffer from a number of inherent problems. In a valve closed position, the upstream pressure on the ball member forces the ball member into engagement with the downstream seat to seal the valve in a virtual check valve type action. Since the pressure on the ball member is substantially responsible for all the sealing force, at low pressure or vacuum conditions, a low sealing force is present and results in a leaking valve with leak paths between the seats and ball and between the outer surfaces of the seats and supporting flanges or shoulders of the opposed end fittings. Elementary seat designs provide no compensation for wear or tolerance errors in the annular seats and thus associated leakage problems are compounded over a period of time.
At conditions where the upstream seat seals against the ball member, the additional surface area of the upstream seat receiving load pressure is imparted against the downstream seat thereby increasing the load upon the downstream seat resulting in increased downstream seat stress and distortion. The force on the upstream seat to its outer diameter surface is translated through the ball member to the downstream seat. Such increased load area results in increased seat stress and thus causes an undesirable wear rate for the downstream seat. The problems of increased seat stress and wear are particularly noticeable on the larger ball valve sizes.
Also at conditions including upstream seat sealing against the ball member at a valve closed position, a problem occurs with a "blowing-in" bulge on the upstream seat at the first opening of the valve. As the valve is being opened, the upstream seat must momentarily span the opening or fluid passage extending through the ball and hold back fluid pressure. During this short period of time until the upstream seat seal is relieved by further opening of the valve, the portion of the seat spanning the fluid passage can deform into the passage under fluid pressure. With a small opening in the ball, the seat is quite rigid when loaded as a beam in bending and can easily bridge the gap. As the valve size and ball opening increase, the section modulus of the seat does not proportionately increase to retain the same stiffness. Thus, the seat may deflect and deform further into the ball opening to form a bulge in that sector of the seat ring.
As repetitive cycling occurs throughout the life of the valve, the bulge in the upstream seat becomes more pronounced and actually operates to cam a floating ball against the downstream seat as the ball member is rotated to the valve open position. The resulting camming action further stresses, even cutting, the downstream seat, and may ultimately distort and wear the seat into a non-operative condition. The bulge on the inlet seat cams the ball off-center and holds it away from both seats while closed causing seat leakage in the closed position. It is particularly noteworthy that the industry focused on the downstream seat assembly for purposes of later improving ball valve operation to prevent valve failures and problems caused by blowing-in bulges. In actuality, however, the upstream seat design was the cause of the problem. The upstream seat permitted the blowing-in bulge and the resultant camming action which was reflected to the downstream seat.
Where soft plastic seats are employed, such as those made of Teflon, particular problems encountered with cold flow creep in the seat at non-contained portions aggravate wear and undesirable seat deformation.
Blowing-in bulge aside, a sealing upstream seat has a generally unsupported annular front face that can creep in toward the center of the valve when the valve is closed and under long duration static fluid pressure drop across the upstream seat. In the elementary seat design the outside diameter of the seat ring is normally supported only by a shoulder in the valve body. The area of the seat and, therefore, the force acting on it, increases as the square of the diameter. Since the supporting shoulder is usually quite narrow and its area is more closely related to the seat circumference, the supporting shoulder width only increases in a linear fashion. This results in a narrow supporting shoulder which allows the seat to shear past it. Ultimately as the seat creeps forward it can cave-in entirely or before that point it typically curls into the orifice of the ball such that when the valve is operated, the seat is torn. This problem also becomes more pronounced as the valve size increases.
The down stream seat can also creep in toward the center of the valve by its generally unsupported annular front face. When the valve is in the closed position, the seat may be displaced upstream by the ball member pushing into it under pressure load.
Where large ball members are employed in large valves, the ball member weight additionally contributes to the deformation and cold flow problems. As the seats deform after a period of use the ball member may sag to the bottom of the valve, thereby providing a clearance at the top of the valve for a leak path.
An evolutionary step beyond the foregoing elementary seat design is the contoured seat in which the ball engaging surface of the seat includes a contour different from that of the ball member outer periphery. The objective of this contoured design is to obtain a narrow band or line contact between the seat and the ball which flexes slightly under loads and retains contact with the ball at low pressure situations to give a sealing stress at low pressure to minimize leakage between the ball member and seat. However, in addition to retaining the other problems associated with the elementary seat design, the contoured seat suffers from the additional problems of extreme contact line distortion in high pressure conditions and rapid wear due to high seat stress since the entire sealing force was substantially borne by the contact line band. The extreme distortion and rapid wear further aggravated the other problems.
In order to overcome the particular wear problems of contoured seat designs, the flexible seat design, as is illustrated in U.S. Pat. No. 2,945,666 evolved. The spring action of the flexible seat provided low pressure contact, wear compensation, tolerance compensation and ball sag compensation. However, this design suffers from the particular problem of providing improved operation for only a short period of time.
The seat is typically constructed of a plastic material which lacks an elastic memory and which deflects under load but does not return to its original shape when the load is removed. In addition, the flexible seat design employs less seat material at the contact line with the ball. After short periods of exposure to high pressure, the flexible seat will distort to the shape of an elementary seat which is severely worn.
The problems of creep at non-contained portions of both upstream and downstream seats, high downstream seat load due to an upstream seat seal, seat-to-shoulder leak paths, and seat blowing-in bulges all remain with both the flexible seat design and the contoured seat design.
The industry next developed a ball valve seat design comprised of a seat having a seat ring with a metal frusto-conical disc spring disposed in operative engagement with the seat ring rear face. The disc spring allows for improved compensation for seat ring wear, distortion and creep as well as seat tolerance and ball sag, while providing an elastic support which effects a low pressure force bias. The disc spring provides seat elasticity not dependent on the elastic memory of the plastic seat material. However, while this is an improvement over the before-mentioned flexible seat design, the disc spring must, nevertheless, provide its elastic force while overcoming the elastic resistance of the plastic seat material.
A particular problem with this design is that the action of the spring assures that the upstream seat seals against the ball member. Thus, the problem of the elementary seat design having an effective load area on the downstream seat substantially equal to the surface of the upstream seat is not mitigated. The remaining unsolved problems include a blowing-in bulge, a weak seal at the seat-to-support shoulder contact area and creep on the non-contained portions of both the upstream and downstream seat rings. In addition, a problem with such design is that the disc spring need overcome the elasticity of the seat ring to effect the low pressure force bias.
In order to meet the problems caused by an upstream or inlet sealing seat, the industry developed an upstream seat bypass arrangement such as is typically illustrated in U.S. Pat. No. 2,930,576. Various upstream seat bypass methods and designs are employed including seat notches, grooves, holes, body orifices, and check valves. These variations are all for the purpose of communicating line pressure around the upstream seat at the valve closed position for relieving the upstream seat ring, for reducing the load area by which the ball member imparts a load on the downstream seat, and thus for reducing seat stress, creep and wear at the downstream seat ring. Also upstream seat blow-in and creep under static pressure is circumvented. In addition, the necessary operating torque for opening and closing the valve is lessened.
Particular problems encountered with the upstream seat bypass designs include the need to provide dynamic seals due to bi-directional sealing of a ball-to-seat seal and a seat-to-body seal. A necessary consequence of such complicated bi-directional sealing is reduced reliability of such elaborate sealing. Problems of low pressure seat leakage, seat-to-shoulder leakage, compensation for tolerance, sag and wear are not addressed.
Plugging of the bypass routes with contaminants in the valve system fluid frequently occurs, especially where the valve is required to operate in a dirty environment. Where a high viscosity fluid is employed a transient delay is often necessary before pressures equalize about the inlet seat thereby allowing the bypass routes to operate correctly. In both cases this seat design then assumes all the problems of an elementary seat design including a blowing-in bulge and cold flow creep at the non-contained portions of the seat rings.
A recent step in the evolution of ball valve designs comprises the provision of a support ring opposite the disc spring to contain the plastic seat ring. The support ring operates to provide additional seat ring support for bridging the ball passage at valve opening to minimize the blowing-in bulge and the associated reflected distortion on the downstream seat. Additionally, the support ring confines the seat to minimize cold flow creep distortion. However, those seat designs which include disc springs and support rings are not without problems. Specifically, as with the before-mentioned disc spring and plastic seat design, the spring reserve of the seat-spring combination is determined by the spring only which actually must overcome the plastic resistence of the seat in order to move against the ball member and compensate for wear, tolerance and ball sag. The support ring in preventing the seat from caving in also hinders the seat from moving forward against the ball. Thus, the seat is actually obstructed from moving against the ball member by the support ring. While the support ring provides support against seat creep and blow-in bulge it holds back the seat and spring combination from compensating for wear, tolerance, and ball sag. As the valve is cycled and the seats are worn, the disc spring forces must overcome both the elasticity of the seat ring and the rigid support of the support ring to effect seat to ball engagement.
It has, therefore, been desired to develop a ball valve and seat assembly for a floating ball type of valve which could be employed with higher fluid system pressures while producing a longer life span than has heretofore been possible. Preferably, such a design would eliminate the necessity for utilizing a trunnion mounted ball as is generally conventional for ball valves employed in elevated fluid system pressure conditions. Trunnion mountings are not considered practical unless the valve is quite large because such mountings substantially increase the size, complexity and cost of the valves.
The present invention contemplates a new and improved construction which overcomes all of the above referred to problems and others and provides a new and improved floating type ball valve and seat assembly which facilitate increased pressure capabilities and extend the effective valve life and wherein the seats may be formed from a wide variety of materials to suit a wide range of operating conditions or parameters.